As car navigation devices have been becoming widespread, the VICS (Vehicle Information Communication System) has come into use, while road traffic information such as traffic jam information has been supplied as VICS information to vehicles. For supplying the VICS information to the vehicles, vehicle-to-roadside communications and FM multiplex communications have been carried out. In the case of vehicle-to-roadside communications, optical beacons and the like are provided on the roadside, while receivers corresponding to the optical beacons and the like are mounted to the vehicles. In the case of an optical beacon, a head for transmitting/receiving data is provided in each lane and downlinks the VICS information common in all the lanes to each lane simultaneously.
It has further been contemplated to utilize optical beacons in order to assist driving by vehicle-infrastructure cooperative services. In an example of the vehicle-infrastructure cooperative services utilizing optical beacons, a vehicle performs position location according to data received from an optical beacon and calculates a distance from the vehicle to a stop line according to the position location. Then, the vehicle acquires signal information and carries out various driving assistances according to the information about the distance to the stop line and the signal information. The signal information includes cycle information about color lights and right-turn lights and the like, while details of information vary among lanes such as straight-through lanes and right-turn lanes. Therefore, for performing the vehicle-infrastructure cooperative services, information must be set for each lane, while it is necessary for the optical beacons to downlink different kinds of information from respective heads for the lanes, so as to provide different services for the respective lanes.
However, a part of a downlink area of a head corresponding to each lane of an optical beacon enters its adjacent beacon. Therefore, when the heads transmit different data for the respective lanes, the data may interfere with each other because of leakage from adjacent lanes, thus causing errors in reception, whereby necessary data may not be received totally. Hence, the communication device described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-182190) transmits common data common in all the lanes to all the lanes and then, during when transmitting lane-specific data to a given lane, stops transmitting data to its adjacent lanes (i.e., performs time-division transmissions between lanes), thereby preventing data from interfering with each other between the lanes. Japanese Patent Application Laid-Open No. 8-86662 also discloses a communication device on the roadside.